“This is the first time cryo-TEM techniques have been used to actually image the dislocation and twinning activity in these alloys in real time at cryogenic temperatures”Rob Ritchie
An international group of researchers have identified a series of different deformation metallic alloys that brings a greater understanding of their exceptional strength, ductility and especially toughness. They were also surprised to discover that, unlike most metallic materials, these properties in both the CrMnFeCoNi and CrCoNi alloys actually improve at cryogenic temperatures.
With high-entropy alloys (HEAs) – multi-principal element metallic alloys – seen as a key area of research in metallurgy, the alloy that has been most studied is the so-called Cantor (CrMnFeCoNi) alloy and its derivatives, such as the CrCoNi alloy, the focus of this study. As described in Materials Today (Ding et al. Mater. Today (2019) DOI: 10.1016/j.mattod.2019.03.001], the team were the first to utilize in situfracture studies by high-resolution transmission electron microscopy (TEM) to understand the mechanisms of deformation in these alloys, and also to explore these deformation mechanisms at such low temperatures.
With the microscopy led by Qian Yu at Zhejiang University in Hangzhou, the theorist Ting Zhu fromGeorgia Tech, and Rob Ritchie’s group at the University of California, Berkeley, in association with colleagues at Oak Ridge, they used cryo-TEM techniques to image the series of deformation mechanisms responsible for the exceptional strength and ductility, which tend to be mutually incompatible properties, in these CrCoNi-based alloys. As Rob Ritchie told Materials Today, “This is the first time cryo-TEM techniques have been used to actually image the dislocation and twinning activity in these alloys in real time at cryogenic temperatures”.
The alloys, and particularly CrCoNo, have one of the best damage tolerance based on a combination of strength and fracture toughness at cryogenic temperatures ever recorded for any material, and the work helped to verify what were believed to be the mechanisms responsible for these properties. It also showed the preponderance of cross-slip as a prime dislocation motion mechanism at these temperatures.
High-entropy alloys, which can be processed as regular metallic alloys, could find applications as structural materials, and there are a huge number of different combinations of elements that remain to be explored. Although new structural materials tend to take many years before they become viable for application, the team believe that due to their damage-tolerance properties some of the alloys will achieve real industrial applications sooner rather than later.
They plan to continue exploring the unique deformation in these alloys with high-resolution TEM as well as mechanical testing techniques, and also to explore refractory high-entropy alloys (RHEAs), which contain combinations of very high melting-temperature elements, to make ultrahigh-temperature materials.
Cryo-TEM images taken at 93 K showing real-time dislocation and twinning events associated with dislocations impinging on boundaries in CrCoNi-based high-entropy alloys. The synergy of dislocation mechanisms leads to both high strength and very impressive ductility, which combine to give these alloys exceptional fracture toughness properties that remarkably can get even better at cryogenic temperatures.